This invention relates generally to ebonite coatings for metal substrates. More particularly, it relates to an ebonite tape.
There is a pervasive and continuing need for protecting metals from corrosive chemical action, such as in metal pipes, stacks, chimneys, bridges, chemical plant constructions, ship hulls, and containers for aggressive chemicals, to name just a few applications. In addition to having a high resistance to chemical action, an ideal coating has certain other properties: the raw materials required to produce the coating are preferably commercially available, inexpensive and non-hazardous; the coating should have the ability to be easily applied to the metal; the coating adheres strongly to many different metals; the coating should be strong, hard, abrasion resistant and thermostable; and the hardening process of the coating can be performed in contact with moisture, does not require extreme or long heating, and does not release toxic fumes. An ideal metal coating may have many additional properties, depending on the particular application or purpose of the coating.
The most widespread anticorrosive coatings possessing many of the above properties are polyurethanes and epoxide resins (see for example, Coating Systems: A guidance Manual For Field Surveyors, American Bureau of Shipping and Affiliated Companies, 1995). These coatings have good chemical resistance to many substances, have adhesion to metals that is satisfactory for many purposes, and have good mechanical properties. Neither polyurethanes nor epoxide resins, however, satisfy all the criteria for an ideal coating for metal. In particular, although polyurethanes have outstanding oil-gasoline resistance, a unique combination of favorable physical-mechanical properties, and strong adhesion to some metals, they are not stable under elevated temperature, alkaline hydrolysis:, and persistent tension. Epoxide resins, although they have outstanding adhesion to some metals, do not have a satisfactory resistance to acids, certain solvents, temperature changes, and vibration. One of the most significant problems associated with both epoxide resins and polyurethanes is their susceptibility to underfilm corrosion associated with defects in the coating surface. Because these coatings are bonded to the metal only by adhesive bonding, these bonds can be broken by the introduction of moisture, solvents or other substances.
As is known from rubber chemistry (Encyclopedia of Polymer Science and Technology, John Wiley and Sons, N.Y., vol 12, p.161, 1970), solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds). The sulfur and polydienes are combined with some auxiliary additives and heated to produce vulcanization. Typical mass ratios of sulfur to rubber are 2:100 for elastomeric rubber and 40:100 for hard rubber. Due to the large degree of sulfide cross-linking formed in the vulcanization process, solid ebonite is a hard, non-flexible, plastic-like material possessed of unique chemical resistance to aggressive substances such as acids, alkalis, salt solutions, oil, and gasoline. In addition, solid ebonite has good mechanical properties. Consequently, these conventional rubbers are commonly used as materials for fuel tanks, containers for aggressive substances, and other applications. In spite of these advantages, however, solid rubbers can not be easily applied to metal surfaces, they release toxic fumes during vulcanization, and they require a long time to harden.
More than 30 years ago liquid rubbers were synthesized. (See Alan R. Luxton, xe2x80x9cThe Preparation, modification and application of non-functional liquid polybutadienesxe2x80x9d, Rubber Chemistry and Technology, 54 (1981) 3, 596-626.) Like earlier rubbers, liquid rubbers are formed from compounds such as polybutadiene, polyisoprene, butadiene-styrene, and butadiene-nitrile. In contrast to the hard rubbers, which are made from such compounds having molecular weights on the order of 100,000 to 500,000, the liquid rubbers are made from such compounds having molecular weights of only 2,000 to 4,000. Consequently, the low molecular weight rubbers permit castable processing by pouring, spreading, spraying, or rolling, while providing similar properties as the hard rubbers after curing. Liquid rubber, therefore, may be used to more easily coat metal surfaces.
Liquid ebonite mixture (LEM) compositions are disclosed by Figovsky in WO 0,006,639 issued Feb. 10, 2000, and liquid rubber based ebonite coating has been disclosed by Rappoport in U.S. Pat. No. 5,766,687 issued Jun. 16, 1998 and U.S. Pat. No. 5,997,953 issued Dec. 7, 1999. The liquid ebonite compositions have advantage of applying, by brushing, rolling or spraying a thin layer to coat any surface with complex geometry, such as bolts and anchors. The liquid coatings can usually stick to the substrate directly without using additional primer or adhesive. However, it is usually inconvenient and messy to handle. Furthermore, the coating is heated by hot air or steam to typically 160xc2x0 C. to 180xc2x0 C. for vulcanization.
Ebonite rubber sheets have been produced for coating metal substrates. The ebonite rubber sheets of prior art are either extruded or calendered to give a relatively thick layer, typically greater than 0.0.625 inch, which is easy to handle and can be cut into any desired shapes. However, due to its thickness and high modulus, the rubber sheets of prior art can only be applied by laying them onto a substrate without stretching. It is usually very craft sensitive to lay and join rubber sheets neatly to provide a complete surface coverage. In addition, such sheets can not effectively coat a substrate with complex geometry, such as bolts and anchors. Furthermore, an adhesive or primer is invariably required to fix and bond rubber sheets onto the substrate. In addition, similar to liquid ebonite coating, the same vulcanization condition is required to cure the laid rubber sheet.
There is a need, therefore, for an ebonite tape that will overcome the disadvantages of the prior art, while maintaining all ebonite key properties, such as chemical resistance and tenacious bonding to metal.
Disadvantages associated with the prior art are overcome by an ebonite tape formed by either a single component mixture or a two-component mixture.
According to a first embodiment of the present invention, an ebonite tape is made by a single component mixture. The mixture includes unsaturated polymers, a vulcanization agent, a vulcanization activator, a solubilizer for the vulcanization agent, and a vulcanization accelerator. Unsaturated polymers can be polybutadiene, polyisoprene, or poly(butadiene-co-acrylonitrile), which contain high amount of unsaturation (typically double bonds) in the backbones for forming linkage with a high loading of the vulcanization agent. The mass parts of unsaturated polymers in the mixture are 100. Unsaturated polymers may or may not contain functional groups, such as hydroxyl, epoxy or acrylic, and may be partially epoxidized.
A preferable vulcanization agent is sulfur, with mass parts of approximately 15-50, preferably 30-50. A solubilizer for sulfur preferably contains polyamine, such as aliphatic, cycloaliphatic, amidoamide, and polyamide amine, with the mass parts approximately 1.5-6. The vulcanization accelerator preferably contains thiuram disulfide, such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, and tetrabenzylthiuram disulfide, and its mass parts is approximately 3-5. The thiuram disulfide will react with polyamine and causes the mixture to gel at ambient condition.
The vulcanization activator may contain zinc oxide or zinc stearate, and its mass parts are approximately 5-35. The mixture further includes a viscosity reducer for adjusting the viscosity of the mixture and the surface tack of the tape. The viscosity reducer can be any liquid rubber, which is compatible with and has a lower viscosity than the unsaturated liquid rubber. Furthermore, the viscosity reducer is reasonably non-volatile so that no significant weight loss occurs during vulcanization condition. The mass parts of the viscosity reducer is approximately 0-30. The single component mixture additionally includes a thixotropic agent of fume silica or bentonite with mass parts approximately 0-10, and a reinforcing agent of carbon black with mass parts approximately 0-10.
The single component mixture is mixed and cast on a release paper, such as silicone release paper, polypropylene release paper, or polyethylene terephthalate release paper, at ambient condition. The mixture gels after four days and turns into a soft elastic tape that typically has a tensile strength at 100% elongation less than 100 psi, and an elongation greater than 50%. An ebonite coating is formed on a metal substrate by wrapping the ebonite tape onto the substrate and baking it at elevated temperature for vulcanization.
According to a second embodiment of the present invention, an ebonite tape is made by a two-component mixture. First component of the mixture includes first unsaturated polymers, a sulfur vulcanization agent, and a vulcanization activator. Selected first unsaturated polymers contain first functional groups, such as hydroxyl, epoxy, acrylic, and their combinations, which are capable of reaction at ambient condition.
Second component includes second unsaturated polymer having second functional groups, such as isocyanate or maleic anhydride, which will react with the first functional groups at ambient condition. Upon mixing the first and second components, the first functional groups will react with the second functional groups at the ambient condition, with or without the aid of a catalyst, to form an elastic network, thereby the two-component mixture gels at ambient condition. By adjusting the molecular weight and the stoichiometry of the first and second unsaturated polymers, desired low modulus, high elongation and surface tack suitable for the tape application are achieved. Furthermore, the second unsaturated polymers must be thermodynamically compatible with the first unsaturated polymers so that the macroscopic phase separations will not occur and the sulfur vulcanization can happen homogeneously throughout the coating.
The first and second unsaturated polymers must contain sufficient unsaturation in the backbones for forming linkages with the vulcanization agent. It is preferable that the polymer backbone is polybutadiene. However, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene), and poly(ethylene-co-propylene-co-diene) can also be used. The mass parts of the first unsaturated polymers in the mixture is 50, and the mass parts of the second unsaturated polymers is approximately 50-100. Alternatively, functional terminated polymers that are partially epoxidized can be used as the first unsaturated polymers. In addition, toluene diisocyanate terminated prepolymers and 4,4xe2x80x2-methylene diphenyldiisocyanate prepolymers are also used as the second unsaturated polymers.
In addition, first and second components may include first and second viscosity reducers for adjusting the viscosity of the mixture and the surface tack of the tape. The first and second viscosity reducers must have lower molecular weight and lower viscosity than the first and second unsaturated polymers respectively. The mass parts of the first and second viscosity reducers are approximately 0-30.
Furthermore, the first component may include a thixotropic agent of fume silica or bentonite, and a reinforcing agent of carbon black. Mass parts of the thixotropic agent and the reinforcing agent are approximately 0-10. The amount of thixotropic agent and reinforcing agent should be minimized so that they will not increase the viscosity of the mixture drastically.
The first component, preferably but not absolute necessarily, further includes a sulfur solubilizer. Preferred sulfur solubilizer is polyamine with the mass parts approximately 1.5-6. In addition as serving as a solubilizer, polyamine is also reactive to the isocyanate groups in the second unsaturated polymers to form polyurea linkages, which accelerates the gelation process. Furthermore, a urethane catalyst is also added into the first component with mass parts approximately 0-3.
A vulcanization accelerator is additionally added into either first or second components. If thiuram disulfide, such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, and tetrabenzylthiuram disulfide, is used as the vulcanization accelerator, it must be added into the second component since it works synergistically with polyamine contained in the first component, which accelerates the sulfur vulcanization, but also causes the first component mixture to gel. Thus, to maintain sufficient shelf life, thiuram disulfide must be separated from polyamine and is mixed into the second component. The mass parts of thiuram disulfide accelerator in the mixture is approximately 1-10. If diphenylguanidine (DGP) is used as a vulcanization accelerator, since DGP can be mixed with polyamine in the first component without causing a shelf life problem, it can be added in the first component. The mass parts of DPG in the mixture is approximately 1-7.
Optionally, both first and second components can contain other commonly used compounding ingredients, such as fillers, plasticizers, tachifiers, antioxidant, antiozonants, surfactants, wetting agents, defoamers, deaerants, antifouling agents, biocides, corrosion inhibitors, and rheology modifiers.
The first and second components are first formed separately by mixing their compositions. The two components are then mixed together with a mass ratio of between about 0.75 and about 2.75. Two-component mixture is then cast onto a release paper at ambient condition. The mixture gel in 10 minutes and turn into a soft elastic tape that has a tensile strength at 100% elongation less than 100 psi, and an elongation greater than 50%. The tape is wrapped onto a metal substrate and is baked at elevated temperature for sulfur vulcanization to form a coating on the substrate.
The above embodiments provide an ebonite tape that is easy to handle and quick to apply while maintaining all ebonite key properties. The inventive ebonite tape has low modulus and high elongation so that it can be stretched during application to provide some compressive force on a metal substrate, and thereby to ensure good contact and adhesion to the metal substrate as well as to itself. The inventive ebonite tape also can be wrapped multiple times to achieve any desired coating thickness.
Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and: without imposing limitations upon, the claimed invention.
An ebonite tape can be made by either a single component mixture or a two-component mixture. According to the first embodiment of the present invention, the ebonite tape is made by a single component mixture. The mixture includes unsaturated polymers, a vulcanization activator, a vulcanization agent, a solubilizer for the vulcanization agent, and a vulcanization accelerator. Unsaturated polymers must contain sufficient diene unsaturation in the backbones for forming linkages with the vulcanization agent. It is preferable that unsaturated polymers contain backbones of polybutadiene. However, polyisoprene, poly(butadiene-co-acrylonitrile) and the like can also be used. Unsaturated polymers may contain no functional groups, or they may contain additional functional groups, such as hydroxyl, epoxy, or acrylic. An example of such unsaturated liquid polymers is Polybd-R45HTLO (supplied by Atofina). A partially epoxidized unsaturated polymers also can be used.
Sulfur is a preferred vulcanization agent. Vulcanization activator can be zinc oxide. The selections of sulfur and zinc oxide are not critical. However, it is preferable that they have fine particle sizes to make mixing easier. Alternatively, zinc stearate can be used as the vulcanization activator. A preferred solubilizer for sulfur is polyamine, such as aliphatic, cycloaliphatic, amidoamine, and polyamide amine. An example of polyamine is Unirez 2140 (supplied by Arizona Chemicals). The vulcanization accelerator preferably contains tetramethylthiuram disulfide (e.g., Methyl Tuads supplied by R.T. Vanderbilt). However, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, and tetrabenzylthiuram disulfide can also be used. Polyamine works synergistically with tertamethylthiuram disulfide to accelerate the sulfur vulcanization that makes the sulfur curing feasible at 80xc2x0 C. Furthermore, the reaction of polyamine and tetramethylthiuram disulfide will cause the mixture to gel at ambient condition.
The mixture further includes a viscosity reducer, which can be any liquid rubber. The viscosity reducer must be thermodynamically compatible with unsaturated polymers. The viscosity reducer should also be reasonably nonvolatile so that no significant weight loss occurs during vulcanization condition, typically between 180xc2x0 C. for 30 minutes to 80xc2x0 C. for three days. Furthermore, the viscosity reducer must have a viscosity lower than the viscosity of the unsaturated polymers.
In addition, carbon black is added into the mixture as a reinforcing agent, colorant and UV stabilizer. Furthermore, the mixture can include a thixotropic agent, such as fume silica or bentonite. The amount of the reinforcing agent and the thixotropic agent should be minimized so that they will not increase the viscosity of the mixture drastically.
Typical compositions of a single component mixture is shown in Table 1. The mass parts of a compound is number of parts by mass of the compound in the mixture.
The mixture is mixed by charging the liquid polybutadiene, sulfur, zinc oxide, tetramethyltiuram disulfide, and optionally fillers, such as fume silica and carbon black, into a metal can. The mixture is subjected to a high shear mixer, commonly known as Cowles or Lightning mixer, operated at 300 to 3000 rpm for 30 minutes. Due to the shear action, the solid powders (i.e., sulfur, zinc oxide, tetramethylthiuram disulfide, carbon black, and fume silica) are evenly distributed and dispersed into the liquid polybutadiene to form a homogeneous viscous liquid. At this stage, the temperature of the mixture reaches about 65xc2x0 C. At this time, polyamine is added into the mixture, followed by non-functional liquid polybutadiene.
The reaction of polyamine with tetramethylthiuram disulfide causes the liquid mixture to gel at ambient condition.
However, the single component mixture has a limited shelf life. The liquid mixture will gel in approximately 15 days after polyamine was added into the mixture. Therefore, it is essential to cast and prepare the ebonite tape right away. The ebonite is formed by spreading the mixture on a release paper, such as silicone release paper, polypropylene release paper, and polyethylene terephthalate release paper, by using a doctor blade to form a thin film of about 0.020 inch thickness and about 3 inch width, at ambient condition. After four days, the film gels into an elastic ebonite tape. Ebonite tape made of single component mixture has a tensile stress at 100% elongation less than 100 psi, and an elongation greater than 50%. Since the tape has low tensile strength and high elongation, it can be stretched during application for coating metal substrate, which provide some compressive force on the substrate and thereby to ensure good contact and adhesion to the metal substrate as well as itself.
When using to coat a metal substrate, such as a sand-blasted steel pipe of 1 inch outside diameter, the ebonite tape is wrapped, by applying slight tension, around the pipe with a average coating thickness of about 0.035 inch. The coated pipe is baked for vulcanization in an air-circulated oven at 150xc2x0 C. for 30 minutes.
In a second embodiment of the present invention, an ebonite tape is formed by using a two-component mixture, which overcomes the limited shelf life problem of the single component mixture discussed above. A first component of the mixture includes first unsaturated polymers. Selected first unsaturated polymers must contain first functional groups, such as hydroxyl, epoxy, acrylic that are capable of reaction at ambient condition, and must have a high amount unsaturation (typically double bonds) in the backbones, such as polybutadiene, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene) or poly(ethylene-co-propylene-co- diene), for forming sulfur linkages during vulcanization. A preferred liquid unsaturated rubber is Polybd-R45HTLO supplied by Atofina.
The second component typically includes second unsaturated polymers, which have second functional groups that can react with the first functional groups of the first unsaturated polymers at ambient condition, such as isocyanate or maleic anhydride, whereby the mixture can gel at ambient condition. For example, if the first unsaturated polymers contain hydroxyl terminated polybutadiene, then an isocyanate terminated liquid rubber can be used for the second unsaturated polymers. Similar to the first unsaturated polymers, the second unsaturated polymers must contain sufficient unsaturation in the backbones for forming sulfur linkages. Upon mixing of the two-components, the first functional groups and the second functional groups will react at the ambient condition, with or without a catalyst, to form an elastic network. The desired low tensile strength, high elongation, and surface tack suitable for the tape application can be achieved by adjusting molecular weight and stoichiometry of the second functional groups to the first functional groups (or the mix ratio of the first and second components).
A vulcanization agent and a vulcanization activator are added into the first component since a hydroxyl terminated liquid rubber is more inert than an isocyanate terminated liquid rubber. The vulcanization activator can be zinc oxide or zinc stearate. Sulfur is a preferable vulcanization agent with the mass parts approximately 15-50, preferably 30-50. It is preferable that sulfur has fine particle size so that the dispersion will be easier. Furthermore, the first component further includes a thixotropic agent of fume silica or bentonite, and a solubilizer for the vulcanization agent. Sulfur solubilizer preferably contains polyamine, such as aliphatic, cycloaliphatic, amidoamine, and polyamide amine. Carbon black is used as a reinforcing agent, colorant and UV stabilizer. Urethane catalyst, such as dibutyl tin dilaurate (DBTDL), can be mixed in the first component.
The first and second components optionally include the first and second viscosity reducers for adjusting the viscosity of the mixture and the surface tack of the tape. The first and second viscosity reducers usually have lower molecular weights and lower viscosity than those of the first and the second unsaturated polymers.
Furthermore, either first or second component includes a vulcanization accelerator. If thiuram disulfide, such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, and tetrabenzylthiuram disulfide, is used as the vulcanization accelerator, it must be added into the second component since it works synergistically with polyamine contained in the first component, which accelerates the sulfur vulcanization, but also causes the first component mixture to gel at ambient condition. If diphenylguanidine is used as the vulcanization accelerator, it can be added into the first component without causing a shelf life problem.
Optionally, both first and second components can contain other commonly used compounding ingredients, such as fillers, plasticizers, tachifiers, antioxidant, antiozonants, surfactants, wetting agents, defoamers, deaerants, antifouling agents, biocides, corrosion inhibitors, and rheology modifiers.
A typical formulation of the two-component mixture is shown in Tables 2a and 2b.
The first component is mixed by charging the hydroxyl terminated liquid polybutadiene, sulfur, zinc oxide and optional fillers, such as fume silica and carbon black, into a metal can. The mixture is subjected to a high shear mixer, commonly known as Cowles or Lightening mixer, operated at 300 to 3000 rpm for 30 minutes. Due to the shear action, the solid powders (i.e., sulfur, zinc oxide, and optional fillers) are evenly distributed and dispersed into the hydroxyl terminated liquid rubber to form a homogeneous viscous liquid. The temperature of the mixture reaches 65xc2x0 C. At this time, polyamine is added into the mixture, followed by the non-functional liquid polybutandiene and the catalyst.
The second component is mixed similar to the first component. Since isocyanate is sensitive to moisture, the mixing has to be careful to minimize air and moisture entrapment into the mixture.
The first and second components are then mixed together with a mass ratio between 0.75 and 2.75 for preparing an ebonite tape. For further information on formulations of two-component mixtures, see copending U.S. patent application Ser. No. 09/724,698 filed Nov. 28, 2000.
The two-component mixture is spread on a release paper by using a doctor blade to form a thin film of about 0.020 inch thickness and about 3 inch width, at ambient condition. The film gels into an elastic ebonite tape in 10 minutes and become a soft elastic tape in four to eight hours. Ebonite tape made of two-component mixture has a tensile stress at 100% elongation less than 100 psi, and an elongation greater than 50%. Since the tape has low tensile strength and high elongation, it can be stretched during application for coating metal substrate, which provide some compressive force on the substrate and thereby to ensure good contact and adhesion to the metal substrate as well as itself.
When using to coat a metal substrate, such as a sand-blasted steel pipe of 1 inch outside diameter, the ebonite tape is wrapped, by applying slight tension, around the pipe with a average coating thickness of about 0.035 inch. The coated pipe is baked for vulcanization in an air-circulated oven at about 150xc2x0 C. for 30 minutes.